1. Field of the Invention
The present invention relates to the recovery of precious metals from carbonaceous ores. More particularly, the invention concerns improved leaching techniques for these ores.
2. Description of the Prior Art
Gold is one of the rarest metals on earth. Gold ores can be categorized into two types: free milling and refractory. Free milling ores are those that can be processed by simple gravity techniques or direct cyanidation. Refractory ores, on the other hand, are difficult to process. Refractory ore resources can consist of ores, flotation concentrates, mill tailings, and other reserves. In the past, refractory ores have required pre-cyanidation treatments to liberate the gold. The difficulty of processing refractory gold ores is attributable to their mineralogy.
A large number of refractory ores consist of ores with a precious metal such as gold occluded in iron sulfide particles. The iron sulfide particles consist principally of pyrite and arsenopyrite. If the gold remains occluded, even after fine milling of these ores, then the sulfides must be oxidized to liberate the encapsulated precious metal and make it amenable to a leaching agent (or lixiviant).
Carbonaceous gold ores represent a unique class of refractory ores. Not only is gold sometimes found encapsulated in sulfide minerals in these ores, but these ores also contain carbonaceous matter that interferes with recovery by cyanidation. Gold in carbonaceous ores, therefore, can be associated with sulfide minerals, carbonaceous matter, and/or siliceous minerals. P. Afenya, Treatment of Carbonaceous Refractory Gold Ores, Minerals Engineering, Vol. 4, Nos 7-11, pp 1043-55, 1991, hereby incorporated by reference. The distribution of gold in these mineral groups can vary considerably from ore to ore.
Researchers have identified the carbonaceous matter in these ores as containing (1) an activated carbon component capable of adsorbing gold-chloride complexes and gold-cyanide complexes from solution, (2) a mixture of high molecular weight hydrocarbons usually associated with the activated carbon components; and (3) an organic acid, similar to humic acid containing functional groups capable of interacting with gold complexes to form organic gold compounds. P. Afenya, Treatment of Carbonaceous Refractory Gold Ores, Minerals Engineering, Vol. 4, pp. 1043-1055, 1991, hereby incorporated by reference; W. Guay, The Treatment of Refractory Gold Ores Containing Carbonaceous Material and Sulfides, Society of Mining Engineers of AIME, 81-34, pp. 1-4, 1981, hereby incorporated by reference.
Carbonaceous matter, can therefore directly or indirectly interfere with lixiviation. Direct interference with lixiviation is ascribed to either occlusion of the gold within the carbonaceous material or formation of a stable gold-carbon complex similar to a chelate. The more common problem with these ores, however, is indirect interference. This occurs when the gold-lixiviant complex formed during lixiviation is sorbed by the native carbonaceous material and, therefore, is no longer available for recovery from solution. This phenomenon is called preg-robbing.
Preg-robbing is frequently associated with the use of cyanide as the lixiviant. However, it also occurs with gold-lixiviant complexes other than aurocyanide.
Certain clay materials such as illite, kaolin, and montmorillonite are also known to preg-robbingly adsorb the gold-cyanide complex. Thus, the degree of preg-robbing exhibited by an ore depends on the amount of carbonaceous matter and preg-robbing clay materials in the ore. As used herein, it should be understood that carbonaceous component and carbonaceous matter also refer to preg-robbing clays, because the preg-robbing properties of these materials are functionally similar to that of the actual carbonaceous matter in the ore.
While preg-robbing is most frequently associated with cyanidation processes, the preg-robbing phenomenon is also known to occur with other gold-lixiviant complexes such as gold-chloride. The inventor has even experienced preg-robbing of gold-thiourea complexes while using a thiourea lixiviant.
Carbonaceous ores vary significantly from deposit to deposit, and even within deposits, in the amount of carbonaceous matter they contain. These ores have been reported to contain from approximately 0.2% carbon to as much as 5% carbon. P. Afenya, Treatment of Carbonaceous Refractory Gold Ores, Minerals Engineering, Vol. 4, pp. 1043-1055, 1991.
If P represents the preg-robbing component of the ore, V represents a valuable mineral component (i.e., gold, silver, or platinum), and G represents the gangue materials in the ore, then preg-robbing may be illustrated by the following general formula: ##EQU1## Wherein V.sub.1 represents the precious metal closely associated with the preg-robbing material in the ores, V.sub.2 represents the precious metal associated with gangue material, V.sub.x represents the precious metal preg-robbingly removed from the lixiviant solution, V.sub.y represents the precious metal-lixiviant complexes remaining in solution, and V.sub.2-(x+y) represents the amount of precious metal remaining associated with the gangue material after lixiviation.
Thus, the amount of precious metal that is associated with the preg-robbing component of the ore after lixiviation is equal to the amount of precious metal originally associated with the preg-robbing component of the ore plus the amount that is preg-robbingly removed from the lixiviant solution (V.sub.x). The amount of precious metal remaining associated with the gangue material (V.sub.2-(x+y)) is equal to the original amount of precious metal (V.sub.2) minus the amount of precious metal dissolved by the lixiviant (V.sub.x +V.sub.y).
A number of techniques have been developed for processing refractory carbonaceous gold ores. These techniques include flotation, blanking, carbon in leach, roasting, chemical oxidation, and bacterial leaching. Roasting and oxidation by chlorination are the two methods that are most developed and applicable for treating carbon-bearing ores. The others may play some role in the future or are often confused with methods for processing carbonaceous ores, even within the mining industry, when they are really more suited to treating refractory sulfidic ores. The various techniques are described below:
1. Flotation and Depression
This method has been employed successfully where small amounts of gold are associated with the carbonaceous matter in the ore. In such circumstances, the carbonaceous matter can be floated off and discarded. The remaining ore is then processed using conventional cyanidation techniques. This technique, however, does not work for ores in which considerable quantities of gold are associated with the carbonaceous component. J. Orlich, J. Fuestenau, & D. Horne, Column Flotation of Carbon at the Royal Mt. King Mine, SME Annual Meeting, Phoenix, Ariz., February 1992.
One mining operation has tried to produce a high grade concentrate for possible shipment to a smelter and a tailing which could be discarded or directly cyanided. W. Guay, The Treatment of Refractory Gold Ores Containing Carbonaceous Material and Sulfides, Society of Mining Engineers of AIME, 81-34, pp. 1-4, 1981. The concentrates contained both carbonaceous materials and pyrite, but exhibited low recoveries of gold.
According to the process disclosed in U.S. Pat. No. 4,585,550, hereby incorporated by reference, a coal fraction containing economically significant concentrations of a desirous mineral value can be recovered from a carbonaceous ore by flotation. However, under this process, gold values contained in the non-floated fractions of the ore are lost; thus, this process can only be used if small amounts of gold are associated with the unrecovered fractions.
Other goldfields have depressed the carbonaceous component of the ore while floating the sulphide minerals and free gold. P. Afenya, Treatment of Carbonaceous Refractory Gold Ores, Minerals Engineering, Vol. 4, pp. 1043-1055, 1991. Again, however, this technique would not be used if the carbonaceous component contained significant quantities of gold.
A common problem with all of the flotation processes, therefore, is that the gold associated with the ore fraction that is to be discarded is lost because it is generally uneconomical to recover. As a result, the tail fraction must contain very small amounts of gold for the existing flotation processes to work satisfactorily. However, the mineralogy of a carbonaceous gold ore deposit is continually changing. Therefore, as the amount of gold associated with the ore fraction that is to be discarded (i.e., the tail) increases, the amount of gold values lost during flotation also increases. Current flotation processes are not flexible enough to compensate for these changes in the mineralogies of carbonaceous gold ores. The present invention overcomes this problem by preg-robbingly concentrating the gold values in the carbonaceous component of the ore prior to flotation.
2. Blanking
Blanking agents are used to passivate the surfaces of activated carbon in carbonaceous ores. The blanking agents work by selectively adsorbing on the surface of the activated carbon preferentially to the gold-lixiviant complexes in solution. Kerosene, fuel oil, and RV-2 (para nitro benzol azo salicylic acid) have been used as blanking agents. This method is not applicable where considerable quantities of gold are associated with the carbonaceous matter. And as explained in U.S. Pat. No. 3,574,600, blanking is also not applicable to ores that contain significant quantities of organic acids as carbonaceous matter. One of the objects of the present invention is to permit the processing of carbonaceous ores regardless of native carbon content and regardless of the amount of gold originally associated with the carbonaceous matter.
3. Activated Carbon or Resin In Leach or Pulp
Activated carbon or resin can be added to leach solutions to preferentially adsorb aurocyanide. This process rests on the principle of using a stronger aurocyanide adsorbent than the carbonaceous matter in the ore. P. Afenya, Treatment of Carbonaceous Refractory Gold Ores, Minerals Engineering, Vol. 4, pp. 1043-1055, 1991. However, this process is not effective when the ore contains large amounts of carbonaceous matter, because native carbonaceous matter has the ability to adsorb gold cyanide complex four times faster than activated carbon. B. J. Scheiner, Relation of Mineralogy to Treatment Methods for Carbonaceous Gold Ores, Society of Mining Engineers, 87-96, pp 1-6, 1987. Furthermore, CIL processes use relatively large carbon particles, whereas the ore is fine ground, so that the added carbon and its adsorbed gold values may readily be separated from the ore after cyanidation by size.
U.S. Pat. No. 4,188,208, hereby incorporated by reference, describes a method of high temperature carbon-in-leach (hot CIL). This method involves subjecting an aqueous slurry of carbonaceous ore to a preliminary oxidation step. Thereafter, the pulp is heated to a temperature greater than 167.degree. F., and the ore leached using alkali metal cyanide concentrations greater than 0.1%. This method when tested on a pilot scale using ore containing 0.3 oz Au/ton ore produced activated carbon loaded to only 15 oz Au/ton and a final tail 0.045 oz Au/ton.
The disadvantages with the hot CIL process of 4,188,208 is that the high cyanide concentrations and high temperatures used in the process require the use of more expensive alkali metal hydroxides as well as the added cost to achieve the high cyanide concentrations and high temperature. Furthermore, it has been documented that, the equilibrium loading of gold onto carbon decreases with temperature. This decrease in loading of the activated carbon further increases the cost of the process, because it means that more carbon must be used and regenerated per unit of gold production.
4. Roasting
This is the current industry standard for simultaneously destroying carbonaceous matter, and simultaneously oxidizing the sulfide minerals, in refractory carbonaceous gold ores. In fact, the majority of recently built pretreatment plants use roasting. In Nevada, four roasters have been put into operation since 1986, and at least one more is in the planning stage.
Modern roasters use a fluidized bed construction and conventional fuel source to heat the ores. Roasting temperatures are usually between 600.degree. and 700.degree. C. After roasting, the ore is separated from dust and off-gasses and then quenched. Following quenching, the oxidized ore can be processed using traditional cyanide extraction techniques.
For any particular ore composition, roasting plants operate in a narrow range of tolerances. Below optimum temperature the carbon in the ore is not oxidized and remains actively preg-robbing. Above the optimum temperature, the gold in the ore becomes increasingly less amenable to cyanidation or other extraction techniques. Because of the degrading gold recovery with higher temperatures, many roasters are operated toward the lower side of the range. Blanking agents are then added to passivate any unroasted carbonaceous matter. Accordingly, roaster efficiency in a plant environment tends to vary widely with variation in feed stock.
For many years roasting was the only reliable method of treating refractory carbonaceous gold ores to produce high gold recovery. In the last two decades, however, the increasing costs associated with roasting has increased the pressure to find alternative methods for treating refractory carbonaceous gold ores. Roasting costs are driven in large part by two factors: energy economics and environmental regulation. Energy sources are used for both heating and process control, such as oxygen injection. As a result, this method is particularly sensitive to fluctuations in fuel prices. Environmental regulation is also a large and growing cost factor in the operation of roasters. The off-gas must be treated to suppress dust and to remove extremely toxic mercury and arsenic compounds and sulfur dioxide. This is often accomplished using electrostatic precipitators and scrubbers. These pollution control technologies, however, are both expensive and difficult to control.
As emission standards become stricter, roasting process costs increase dramatically. Almost without exception, both analytical studies and actual operators estimate the cost of roasting to be in the area of $10 to $20 per ton of ore, although one source claims an estimate for a proposed plant of $8 per ton.
5. Chemical Oxidation
Currently, hydrometallurgical methods for treating refractory gold ores strongly attract research and development activity. Currently, there are three aqueous oxidation techniques being given attention: (1) chlorine oxidation, (2) autoclave leaching and (3) bioleaching. Bioleaching is discussed separately.
a. Chlorination
This was the method most favored until process economics and environmental regulation tipped the scale in favor of roasting. At least two chlorination plants were operating recently, although one of them may already be off line.
In this process, the ore is ground and mixed with water to form a slurry. Chlorine gas is pumped into the slurry under pressure at a rate of about 60 to 120 lbs/ton, depending on the residence time, organic carbon concentration in the ore, and percent solids in the slurry. The chlorine gas will oxidize the carbon in the ore, rendering it less preg-robbing. After treatment, the hypochlorous acid generated must be treated with a reducing agent to prevent it from destroying the cyanide used later in the process.
This process is particularly sensitive to the amount of sulfide in the ore, because sulfur is oxidized before carbon. Higher sulfide ores require much more chlorine gas. For very refractory ores the "Double Oxidation" process described in P. Afenya, Treatment of Carbonaceous Refractory Gold Ores, Minerals Engineering, Vol. 4, pp. 1043-1055, 1991, hereby incorporated by reference, has been used.
Environmental factors also play a large part in driving costs. Gas emissions from the tanks must be captured by alkaline scrubbers before being released to remove the chlorine they contain. High pressure chlorine gas is extremely dangerous.
Finally, the process is difficult to control in operation, and plants suffer from the corrosive gas. As a result of all of these factors, roasting will be the economically favored alternative to chlorine based oxidation for the foreseeable future.
As a variant of chlorination, NaOCl can be substituted for chlorine gas as the oxidizing agent. Furthermore, NaOCl can be produced in situ by electrolyzing NaCl. The NaOCl is used in the same manner as the chlorine above to oxidize sulfides and carbonaceous matter in the ore. However, the initial capital investment for this technique is high, and unless there is a radical decrease in energy costs, this method will remain even less economically attractive than chlorination.
b. Pressure Autoclaving
This method is far more successful at oxidizing sulfidic materials that make the ore refractory than it is at oxidizing carbonaceous matter that may be present. It is mentioned here for the sake of completeness. A pressure autoclaving process followed by CIL is taught in U.S. Pat. No. 4,552,589, hereby incorporated by reference.
6. Bioleaching
This is the latest process being developed to treat refractory sulfide and carbonaceous gold ores. The process uses bacteria to biologically degrade sulfide minerals and liberate precious metal values so that they can be recovered by conventional technologies. The most widely used and studied bacteria for this process is Thiobacillus ferrooxidans. Bioleaching, however, has little effect on the preg-robbing characteristics of an ore. Therefore, carbon-in-leach or blanking has been used in addition to bioleaching to obtain satisfactory gold yields from carbonaceous ores. Furthermore, it takes days rather than hours to treat the ore.
Thus, since the mining of low grade carbonaceous gold ore began more than 40 years ago, the mining industry has repeatedly tried to find alternative methods of treating carbonaceous ore. These methods have all involved trying to eliminate or block the preg-robbing effect of these ores so that a traditional cyanide process could be used to recover the precious metal values from the ore. The inventor's process is a completely novel approach in which the heretofore deleterious preg-robbing characteristic of carbonaceous ores is used advantageously to concentrate the precious metal values in the carbonaceous ore on the preg-robbing component of the ore for subsequent recovery.
At present, there are large amounts of both located carbonaceous deposits and stocks of mined carbonaceous ore that have been set aside because they cannot be processed economically using current methods.